HPAs and POAs are thoroughly described in Heteropoly and Isopoly Oxo-metalates, Pope et al, Springer-Verlag, New York, 1983, as well as the related applications identified above. The latter applications also describe their use in alkane oxidation.
In order to clarify the terminology used in the art, consider first a specific precursor used in our invention, H.sub.3 PW.sub.12 O.sub.40. Since the cations in this material are hydrogen, the compound is a heteropolyacid. If the cations are not hydrogen but are metals such as an alkali metal, potassium, sodium, or lithium, or are ammonium, as in K.sub.3 PW.sub.12 O.sub.40 or (NH.sub.4).sub.3 PW.sub.12 O.sub.40, then it is obviously no longer an acid, and is referred to as a polyoxoanion.
As described in Pope, HPAs and POAs are cage-like structures with a primary, generally centrally located atom(s) surrounded by the cage framework which contains a plurality of other metal atoms, the same or different, bonded to oxygen atoms. Since the central atom is different from the other metal atoms, it is described as "hetero". The other metal atoms are transition metals and have oxygen bonding such as ##STR1## where four of the singly bonded oxygen atoms are bonded to other M atoms in the framework and the fifth is bonded to the central hetero atom.
A three-dimensional representation of a typical HPA is shown below ##STR2##
This twelve-cornered polyhedron structure is the metal atom configuration of a typical HPA described above. Between any two metal atoms is an oxygen atom, not shown, and from each metal atom is a doubly bonded oxygen, also not shown, and each of the metal atoms is bonded through oxygen to the central metal atom, not shown.
Our invention relates to HPAs (and POAs thereof) having the general formula H.sub.e (X.sub.k M.sub.n O.sub.y).sup.-e where X, the central atom is preferably phosphorous, but others such as antimony, silicon, and boron are also suitable as are the generally known central atoms of HPAs, i.e., the group IIIA-VIA elements. Subscript k is preferably 1 but can be as high as 4-5. M is one or more transition metals (thus the HPA can be referred to as a transition metal HPA), usually molybdenum or tungsten, and n will vary from 5-20. Subscript y is usually 40 but can be as low as 18 or as high as 62. Notation e is the valence of the (X.sub.k M.sub.n O.sub.y) group and will vary from case to case, but e is always the number of H atoms needed to balance the formula. In a preferred HPA, k=1, n=12 and y=40 as in H.sub.7 PMo.sub.8 V.sub.4 O.sub.40. These and similar HPAs are described in the aforesaid related applications. Most of our catalysts are of the Keggin structure and its isomers, as described in Pope, but other structures such as the Dawson structure also are suitable.
The first-mentioned related application describes the improved catalytic activity achieved by incorporating vanadium, titanium, niobium or rhenium as a portion of the M atoms in the formula described above. Thus, H.sub.9 PW.sub.6 V.sub.6 O.sub.40 is significantly more active than H.sub.3 PW.sub.12 O.sub.40 and K.sub.9 PMo.sub.6 V.sub.6 O.sub.40 is more active than K.sub.3 Mo.sub.12 O.sub.40. The number of metal atoms replaced with the promoter can be as many as 12, preferably 6-8, but can be as low as 3-6 or even 1. That application also discloses the introduction of azide into the HPA or POA, as another means of promoting the catalyst.
The other related application also discloses improving the catalytic activity of the basic HPA or POA by replacing an M.dbd.O in the framework around the central or principal metal atom with a different transition metal atom. Thus instead of (O--).sub.5 M.dbd.O at a specific site in the framework, we might have (O--).sub.5 M.sub.1 where M and M.sub.1 are different metals. In terms of formula, if an Mo.dbd.O site in K.sub.3 PMo.sub.12 O.sub.40 is replaced with chromium, we have K.sub.4 PMo.sub.11 CrO.sub.39.
Since HPAs are acids, they have the usual properties of acids which sometimes makes their use preferable to POAs. Such an instance might arise when the oxidation is carried out in an organic solvent such as acetonitrile, for the HPA is more soluble therein than the POA. If the POA is treated with HCl to convert it to the HPA the characteristic HPA structure is sometimes degraded. Our invention avoids this problem.